This invention relates in general to screwdrivers for driving Phillips-type socket screws. These type of screwdrivers are also known as Cross head, star, Pozidriv and Reed and Prince. In particular this invention relates to a screwdriver with an improved blade configuration.
Referring to FIG. 1, a prior art Phillips or cross-point, No.2 screwdriver 11 has a drive axis 13 and a bit 15 at its distal end. Bit 15 has four orthogonal blades 17 that are each tapered at approximately 26 degrees or less relative to drive axis 13. The tip 19 of bit 15 is further tapered from the blades 17 to form a conical point. Screwdriver 11 is sized for driving socket screws such as the No.2 screw 21 shown, although screwdriver 11 could also drive No.1 or No.3 screws as well. Screw 21 has an axis 23 and a countersunk head 25 with a slotted socket 27. When viewed from above, slotted socket 27 appears as a cross or addition symbol that is well suited to be driven by the orthogonal blades 17 of screwdriver 11. Socket 27 has an axial depth of approximately 0.125 inches into head 25, although screwdriver 11 penetrates only about 0.100 inches therein.
The tapered inner surfaces of socket 27 are inclined at an angle that is approximately 26 degrees or more relative to axis 23. The difference in tapers between blades 17 and socket 27 allow screwdriver 11 to be easily inserted and removed from screw 21 during operation. However, the taper difference also forces screwdriver 11 to contact screw 21 exclusively along its socket threshold region, indicated by brackets 29. Since contact points 29 are unsupported above the top surface of screw 21, socket 27 will tend to wear over time and may even become stripped out and unusable. Also note that the deep penetration of screwdriver 11 into screw 21 and the similarities of their tapers prevent the axis 13 of screwdriver 11 from being tilted more than about one degree off axis 23 of screw 21 (i.e. two degrees total in any plane). Sometimes it is not possible to be within one degree due to obstructions, making it difficult to properly drive screw 21.
Phillips head type screwdrivers are also known to xe2x80x9ccam-outxe2x80x9d of the screw socket while in use. The term xe2x80x9ccam-outxe2x80x9d is known in the art and refers to the tendency of the screw blades when under increasing torque to move out of the screw slot. This may occur in a violent manner, injuring the hands of the user and damaging the screw socket, thereby making it difficult for repeat applications of a screwdriver.
A Phillips-shaped screwdriver has a cylindrical metal shaft having an axis coaxial with an insertion direction for driving a screw having a socket. The bit section is formed on a distal end of the shaft and has a diameter equal to the shaft, tapering to a conical portion. The bit section has a first circumferential groove formed in the exterior. Blades are formed on the bit section and consequently each blade has a groove extending across its width that is in a common plane with each other blades groove. A distal end of the first groove is substantially axial where it intersects the bit section. When the screwdriver engages the screw socket, the conical portion inserts to a depth which allows the circumferential groove to substantially align with a top portion of the socket, and a surface area on the bit section engages an interior surface area of the socket for transmitting torque receiving surfaces. The circumferential groove provides a clearance between the bit section and socket top. This design provides a wobble of four degrees total. A variety of shapes may be selected for the circumferential groove. Typically, convex, concave, flat, straight, and v-shaped grooves have been chosen, but any combination of the above may be selected to obtain desired characteristics relating to xe2x80x9ccam-outxe2x80x9d and a force required to pull it from the screw socket while under torque load. The number of grooves provided on each blade of the bit will vary with the number of different size screws that can be driven by that bit. In another embodiment, the bit section is formed on a distal end of the shaft and has a diameter equal to the shaft tapering to a frustoconical portion. In this design the amount of wobble may be up to six degrees total.